WHAT WOULD CAUSE A CYLINDER 1 2 AND 3 MISFIRE ALL AT THE SAME TIME?

1998 Ford Explorer • 173,000 miles

For awhile my explorer has had a very up and down idle but has run great. Today it didnt want to start and then it did and ran fine. I got home and the check engine light came on I checked the codes and it came up as cylinder 1 2 and 3 misfire. What would be the most likly cause for this? It has brand new plugs and wires.